This invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient's body lumen, such as a blood vessel, to maintain the patency thereof. These devices are useful in the treatment and repair of atherosclerotic stenoses in blood vessels and particularly in coronary arteries.
Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway there-through.
A variety of devices are known in the art for use as stents and have included balloon expandable stents made from tubes; coiled wires in a variety of patterns that are expanded after placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self expanding stents inserted in a compressed state and shaped in a zig zag pattern.
Although stents have been used effectively for some time, the effectiveness of a stent can be diminished if it is not properly implanted within the patient's body lumen. For example, a stent which is expanded to a desired final diameter by a balloon catheter may experience non-uniform radial expansion along its axial length due to the increased resistance to radial expansion the stent imposes about the mid-section of the balloon. Consequently, the balloon initially inflates at the proximal and distal balloon ends adjacent the balloon taper, along a path of least resistance, to form toroidally shaped lobes abutting the ends of the stent in a "dog bone" fashion. As the balloon ends over-inflate to form the characteristic "dog bone," radially outwardly acting forces from the balloon interact with the stent structure to radially expand the proximal and distal ends of the stent before the corresponding central section of the stent begins to expand. Continued uneven inflation of the balloon thereby imparts a generally hyperbolic shape to the stent structure extending along the axial length of the stent as the stent ends expand before the corresponding central section. As a result, the stent ends expand to the desired final diameter and contact the vessel wall before the corresponding center section fully expands. This non-uniform expansion often causes the stent ends to slip, relative to the underlying balloon, toward the axial center portion of the stent, thereby contracting the overall length of the deployed stent. Axial contraction of the radially enlarged stent reduces the length of the diseased vessel segment supported by deployed stent structure. Furthermore, when the respective stent ends contact the interior surface of the vessel walls, the center section must continue to expand to reach full deployment. Continued expansion of the stent center drives the stent ends radially upward and axially outward to embed deeper into the relatively soft intima of the arterial wall. This may result in damage to the ends of the stent, injury to the arterial wall, and may cause the stent to be improperly implanted.
Another difficulty encountered using prior art stents involved maintaining the radial rigidity needed to hold open the artery while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery.
Another problem area has been the limiting range of expandability. Certain prior art stents expand only to a limited degree due to the uneven stresses created upon the stents during radial expansion. This necessitates stents with a variety of diameters thus increasing the cost of manufacture and inventory. Additionally, having a stent with a wider range of expandability allows the physician to redilate the stent if the original vessel size was miscalculated.
Another problem with the prior art stents has been contraction of the stent along its longitudinal axis upon radial expansion of the stent. This can cause placement problems within the artery during expansion such as the stent being implanted several millimeters away from the target site (i.e., the site within the vessel or artery to be treated or repaired).
Various means have been described to deliver and implant stents. One method frequently described for delivering a stent to a desired intraluminal location includes mounting the expandable stent on an expandable member, such as a balloon, provided on the distal end of an intravascular catheter, advancing the catheter to the desired location within the patient's body lumen, inflating the balloon on the catheter to expand the stent into a permanent expanded condition and then deflating the balloon and removing the catheter.
What has been needed and heretofore unavailable is a stent which is capable of controlled radial expansion along its entire length, when deployed with a balloon-catheter, thereby allowing the central section of the stent to expand to a desired final diameter to contact the interior walls of the body lumen before the corresponding end sections fully expand, thus ensuring more uniform stent implantation. At the same time, the stent should have a high degree of flexibility so that it can be advanced through tortuous passageways and can be radially expanded over a wide range of diameters with minimal longitudinal contraction. The present invention satisfies these needs.